1. Field of the Invention
The invention concerns an electrohyraulic control valve arrangement for controlling the pressure medium input to and discharge from a linear or rotatorial hydraulic motor, with a main control valve constructed as a three position valve, which includes a piston which is displaceable in alternative directions between end positions in a bore of a housing, which correspond to the maximal values of unrestricted or unblocked cross-section of flow-through paths of the main control valve in alternative functional positions I and II, which steadily increase essentially with increasing displacement of the piston out of a (functionally neutral) center position 0, and is correspondingly reduced with a nearing of the piston to its center position, wherein the piston displacement is controllable via an electrohydraulic servo control valve, which is guided by an electrical setpoint value, by the alternating application and relieving of pressure in two control chambers of the main control valve.
2. Description of the Related Art
An electrohydraulic control valve arrangement of this type is the generally known proportional valve ("The Hydraulic Trainer", Vogel-Publishers, Wurzburg, 1.sup.st Edition 1978, pages 143-145), which in a typical construction includes a main valve constructed as a 4/3-way valve and two pressure regulating valves each of which include via respectively a proportional magnet, which produces an operating force in proportion to electrical strength, controllable pressure regulating valves as servo valve arrangements, via which pressure can be directed to and released from the control chambers of the main control valves.
As a result of its already discussed construction, the known proportional valve is associated with at least the following disadvantages:
On the basis of the always present friction between the magnet anchors and the housing elements of the pressure regulating valve their relationship is associated with a hysteresis, so that a defined value of the energizing current can not always in a predictable manner be associated with a specific opening cross-section of the main control valve. This type of frictional effect brings about a strengthening of the follow or tracking error, or lag, between the setpoint position and actual position of the main control valve piston, as the time interval is reduced, with which the energizing current changes the servo valve, in order to achieve a desired transient response of the respective valve adjustments or positions. Although extreme consequences of such hysteresis-effects can in some respects effectively reduced or avoided thereby, that the time period of the actuating current of the respective controlled proportional magnet is varied, in such a manner, that its temporal middle value corresponds to an effective current strength, which is associated with the desired anchor displacement, which again leads to a servo or pre-control pressure, which should produce a desired positioning of the main control valve piston. The anchor of a servo valve controlled in such a manner is thereby continuously kept in movement, so that the value of the static friction, which is greater in comparison to the sliding friction practically does not come into effect and insofar as the displacement of the controlled anchor continuously under the more favorable or effective secondary condition of sliding friction is possible. Likewise, also with this type of pre-controlling or servo controlling it must be taken into account, that the actual position of the main control valve piston accomplishes only relatively sluggishly the "middle value" of the energizing current of the proportional magnet predetermined setpoint value, since essential agreement of setpoint and actual position can only be achieved after a certain period interval of the periodic energizing or activating current changes, since these are imprinted by superimposing on an alternating current varying between extreme values a direct current which correlates to the setpoint value position to be steered or controlled. For the period duration of the "dither" current utilizable in practice, which acts upon the periodic position change of the servo valve-anchor, of 10 to 20 ms, this means, that-time-wise determined--equilibration of actual and setpoint value the position of the main control valve piston can be achieved only after approximately 1/20 to 1/5 second, which for numerous requirements is too long. This is in particular true then, when the dither-amplitude of the activation current is comparable to the medium value required for adjustment or setting of a pre-determined opening cross-section of through flow-path of the main control valve, that is, in cases in which the main control valve must be operated with relatively small opening cross-sections of its flow through path.